1. Field of the Invention
The present invention relates to laser velocity interferometry, specifically to an improved laser velocity interferometer for measuring large changes in velocity of any reflecting surface.
2. Discussion of Prior Art
Shock experiments involving projectile impacts or explosive detonations have been used for several decades to determine material properties under dynamic conditions and at extremely high pressures. A review of this field can be found in the paper by L. C. Chhabildas and R. A. Graham, "Developments in Measurement Techniques for Shock-Loaded Solids," in Techniques and Theory of Stress Measurements for Shock Wave Applications, Edited by R. B. Stout, F. R. Norwood, and M. E. Foumey, American Society of Mechanical Engineers, AMD--Vol. 83, pp 1-18 (1988).
One of the most valuable instrumentation techniques in shock experiments has been laser velocity interferometry, in which laser light is focused onto the specimen's surface. Some of the reflected light is collected, and, as the surface moves during a shock experiment, the Doppler shift of the reflected light is measured in an interferometer. The continuous measurement of the Doppler shift results in a continuous velocity history of the surface of the shocked specimen, from which, together with other information, the specimen material properties are calculated, as described in the above-mentioned paper by Chhabildas and Graham.
In addition to shock experiments, laser velocity interferometry has been widely applied to the measurement of the velocity histories of foil surfaces which are very rapidly accelerated by the sudden vaporization of a substrate layer, such as by electrical energy deposition or laser light energy deposition in the substrate material. Continuous velocity histories of projectiles as they accelerate through gun barrels have also been obtained through laser velocity interferometry.
A review of the field of laser velocity interferometry can be found in the paper by L. M. Barker, "Velocity Interferometry for Time-Resolved High-Velocity Measurements," which appears in Proceedings of SPIE 27th Annual International Technical Symposium and Instrument Display, San Diego, Calif. Aug. 21-26, 1983.
The most common form of laser velocity interferometer used for the above measurements has been the so-called Velocity Interferometer System for Any Reflector (VISAR), which was originally developed at Sandia National Laboratories by L. M. Barker and R. E. Hollenbach in the early 1970s. Our first technical paper on the VISAR was L. M. Barker and R. E. Hollenbach, "Laser Interferometer for Measuring High Velocities of Any Reflecting Surface," Journal of Applied Physics, Vol. 43, No. 11, pp 4669-4675, November 1972. In the following, the term VISAR will include the laser velocity interferometer which produces good fringe contrast even when illuminated by light from a diffusely reflecting surface, plus any optical elements necessary to make the input signal light into a beam suitable to traverse the optics of the interferometer, any optics both within and outside of the interferometer which are necessary for producing fringes in quadrature, any optics which are involved in producing the required delay time in one of the light paths of the interferometer, any optics to direct the output signal light to light detectors, and any incidental optical elements to direct, shape, filter, or adjust the intensity of the light beams which traverse the aforementioned components. Any optical mounts, frames, adjusting devices, etc., associated with the aforementioned components are also considered to be included in the term VISAR.
The VISAR works by using a beamsplitter to split the incident light beam, which is composed of light reflected from a specimen surface, into the two light paths (legs) of an interferometer. At the end of each leg is a mirror or retroreflector, which reflects the light back toward a recombining beamsplitter, usually a different point on the same beamsplitter that originally split the incident light beam into the two legs of the interferometer. In the following, the interferometer leg length will mean simply the distance from the beamsplitter surface (assuming the same beamsplitter both splits and recombines the beams) to the mirror or retroreflector of the interferometer leg in question.
The legs of the interferometer have unequal light travel times before the two split-off light beams are recombined, but in spite of the different travel times, they meet the criteria for forming high-contrast fringe patterns even when the interferometer is illuminated by light from a diffusely reflecting surface. In meeting these high-contrast fringe criteria, the original VISAR made use of one or more precision glass plates, called delay etalons, in one of the legs (the delay leg) of the interferometer, as explained in the above paper by Barker and Hollenbach. The time difference between the light travel times in the two legs of the VISAR interferometer is called the VISAR's delay time. It is proportional to the total glass thickness traversed by the light beam in the delay leg of the interferometer, assuming there is no glass in the other leg (the reference leg). If both legs of the interferometer contain glass through which the light must pass, the delay time is proportional to the excess thickness of glass through which light must travel in the delay leg.
The time delay arises from two sources: (1) Light travels slower in the glass, and (2) the delay leg with the excess glass must be physically lengthened in proportion to the amount of excess glass in order to meet the criteria for obtaining good fringe contrast. The precise distance by which the delay leg must be lengthened because of the addition of a given delay etalon is called the etalon's incremental delay leg length. Likewise, the precise time by which the VISAR's delay time is lengthened because of the addition of a given delay etalon is called the etalon's delay time.
The optical components of the interferometer which are physically moved in order to change the delay leg length are called the movable optical components of the interferometer. Similarly, those components which do not move enough to significantly affect the delay leg length are called the fixed optical components of the interferometer.
The specimen velocity change which would result in a fringe shift of one fringe in the VISAR interferometer is called the VISAR's Velocity-Per-Fringe (VPF) constant. The VPF constant is inversely proportional to the VISAR's delay time, and is a measure of the sensitivity of the VISAR. VISARs are usually made to allow for changing the sensitivity by adding or subtracting delay etalons in the delay leg of the VISAR. We shall refer to such VISARs as multi-etalon VISARs. The sensitivity (the VPF) of a multi-etalon VISAR can be changed to best fit the needs of a particular experiment.
When a VISAR interferometer is properly aligned, the output signal beams will normally illuminate only the central "bull's eye" of the interferometer's fringe pattern. Thus, only a small part of a fringe will be visible at any one time, and a fringe shift of one fringe will appear as one complete cycling of the light intensity.
The light fringes produced by a VISAR in a velocity measurement are normally recorded using photodetectors, such as photomultipliers, to transduce the fringe light intensity variations into voltage variations. Digitizing oscilloscopes may be used to record the voltage variations as a function of time. The voltage-time data points collected by the digitizing oscilloscopes can then be analyzed in a data reduction computer program to obtain the velocity vs. time which the specimen surface experienced during the experiment. Streak cameras have also been used to record the VISAR fringe shifts during an experiment.
VISARs also normally use polarization coding to obtain two sets of fringes approximately 90.degree. out-of-phase with each other. This greatly enhances the accuracy of the data, allowing the fringe count to be determined at any time to about plus or minus 2% of one fringe, such that a data record containing four fringes can be expected to be accurate to within 1/2% of the peak velocity. The polarization coding also allows one to distinguish acceleration from deceleration.
The original multi-etalon VISARs had these attributes:
(1) Variable sensitivity to fit the experiment, by varying the delay time, PA1 (2) The ability to measure any surface, whether specular or diffusely reflecting, PA1 (3) Polarization coding for accuracy and for distinguishing acceleration from deceleration, PA1 (4) Fringes in proportion to velocity, not displacement, greatly decreasing the frequency response required to acquire the data, as well as decreasing the complexity of the data reduction, PA1 (5) Nanosecond time resolution, PA1 (6) Better than 1% accuracy in most experiments, and PA1 (7) Absence of any perturbation (by the instrumentation) of the velocity being measured. PA1 1. The fringe alignment of the interferometer is preserved, even in the presence of normally misaligning events such as changing of its installed delay etalons or moving of the VISAR from one location to another. PA1 2. The instrument's size and weight are much smaller than previous multi-etalon VISARs. PA1 3. The sensitivity (i.e., the delay time) is much more easily changeable than in previous multi-etalon VISARs. PA1 4. The thermal and mechanical stability of the instrument are increased, minimizing the need for frequent optimization of the fringe alignment.
A 1976 paper by B. T. Amery, "Wide Range Velocity Interferometer," in Sixth Symposium on Detonation (Office of Naval Research, Dept. of the Navy, Arlington Va., Aug. 24-27, 1976), pp. 673-681, called attention to the fact that the delay etalons in a VISAR interferometer can be replaced by two lenses which are separated by the sum of their focal lengths, thus achieving the required delay while retaining the diffuse specimen surface capability. A much wider range of delay times is available with the lens-generated delay leg, which allows for the accurate measurement of much smaller velocities when long delay times are selected. The intrinsic rise-time of the VISAR is not considered to be less than the delay time, however, so the advantage of finer velocity resolution carries the penalty of a slower rise-time. The present invention relates to multi-etalon VISARs, rather than Amery's lens delay leg VISARs.
A very significant improvement to the multi-etalon VISAR was made by Hemsing in 1978, and published in his paper Willard F. Hemsing, "Velocity Sensing Interferometer (VISAR) Modification," Review of Scientific Instruments, Vol. 50, No. 1, pp 73-78, 1979. The improvement cuts the amount of required laser light by at least 50% without any sacrifice in the signal-to-noise ratio of the instrument by making better use of the light emerging from the VISAR interferometer. In addition, stray non-laser light which may find its way into the signal light beam, such as self-light generated by the experiment, is largely self-cancelling. Hemsing's improvement retains all of the above listed attributes.
Because of the multi-etalon VISAR's impressive list of attributes, it has become widely recognized as the instrumentation technique of choice in certain applications requiring accurate measurement of large velocity changes.
Nevertheless, the use of multi-etalon VISAR instrumentation has been hampered by the very precise alignment requirements of the reflecting surfaces in the VISAR interferometer before its light fringes become visible. Even after visible fringes are obtained, previous multi-etalon VISAR embodiments often require frequent fringe optimization because of thermally or mechanically induced drift in the exact positions of the interferometer's reflecting components. Further, the fringe alignment is nearly always lost completely on changing delay etalons or on moving the VISAR from one location to another, for example. Once fringe alignment is lost, finding fringes again can be quite time consuming. Alignment difficulties can be very frustrating for the individuals setting up experiments, and they lead to inefficient use of time.
Another disadvantage of previous multi-etalon VISARs is their relatively large size and weight, which greatly limits their portability and requires large laboratory areas for their use.
The size, weight, and alignment problems are addressed in U.S. Pat. No. 5,245,473, granted to Philip L. Stanton, William C. Sweatt, O. B. Crump, Jr., and Lloyd L. Bonzon for "Apparatus and Method for Laser Velocity Interferometry." In their invention, the ability to change the sensitivity of the VISAR to fit the experiment is sacrificed in order to obtain the goals of small size and weight, plus fringe alignment stability. This is accomplished by permanently fixing a single delay etalon in the delay leg of the interferometer, and by cementing that etalon and all of the formerly movable interferometer components together in a pre-aligned configuration. This greatly simplifies the alignment problem, because fringe alignment is never lost; it only needs to be optimized slightly by piezoelectric means before a measurement.
Unfortunately, however, attribute No. 1 above, the ability to adjust the multi-etalon VISAR's sensitivity to fit the experiment, is lost in the single etalon "fixed cavity VISAR" of the Stanton, et. al., invention. Inasmuch as the entire interferometer, including a delay etalon, is permanently cemented together into one piece, it is impossible to change the delay, and thus the sensitivity, in the Stanton, et al. fixed cavity VISAR. The fixed cavity VISAR is therefore well suited to quality control types of measurements where nearly the same velocity range is experienced in every test, but it is not well suited for research experiments which involve very different velocity ranges.